Recently, with exhaustion of fossil fuels and concern about environmental pollution, there is an increasing interest in electric vehicles or hybrid vehicles using electrical energy, but not using fossil energy.
To run the electric vehicles or hybrid vehicles, a driving motor requiring a high output should be operated. For this purpose, batteries for the electric vehicles or hybrid vehicles use, as a power source, electricity outputted from a battery pack including a plurality of battery cells connected in series.
A plurality of battery cells included in the battery pack need to individually maintain a uniform voltage for ensuring stability, a long life span, and a high output.
A method for uniformly balancing the charging voltages of battery cells includes increasing the voltage of a battery cell having a relatively low voltage by supplying a charging current, decreasing the voltage of a battery cell having a relatively high voltage by discharging the corresponding battery cell, discharging a battery cell having a voltage higher than a target balance voltage set based on the voltage of each battery cell and charging a battery cell having a voltage lower than the target balance voltage, and the like.
These cell balancing methods are implemented by a cell balancing circuit connected to each battery cell. The cell balancing circuit includes a switching circuit for controlling the start and end of a cell balancing operation, and a discharging resistor for discharging a battery cell.
However, when an abnormality occurs during a cell balancing operation using the cell balancing circuit, for example, when overcurrent instantaneously flows in the cell balancing circuit, or when overvoltage higher than an operating voltage is applied to a switching circuit, or when overheat is generated through a discharging resistor, a component included in the cell balancing circuit is short-circuited or open-circuited, and consequently, the cell balancing circuit is not operated normally.
When the cell balancing circuit abnormally operates due to this problem, the voltage of a battery cell connected to the corresponding cell balancing circuit excessively increases or decreases, and as a result, severe results may be produced, for example, a battery pack may explode or the operation of a load connected to the battery pack may suddenly stop.
To overcome this drawback, there is a need of a separate diagnosis circuit connected to a cell balancing circuit for diagnosing an abnormality in the cell balancing circuit.
For example, Japanese Patent Publication No. 2007-085847 (conventional art) discloses an apparatus for detecting an abnormality in a cell balancing circuit, in which the cell balancing circuit including a field effect transistor (FET) and a discharging resistor, and a resistor interposed between the source and the drain of the FET are installed at each battery cell, a difference in voltage between the source and the drain is measured through the resistor using two comparators applied with different levels of reference power sources, and an abnormality of the cell balancing circuit is determined based on the measured voltage level (high, low).
However, the conventional art needs a separate circuit configuration, that is, the diagnosis circuit for detecting an abnormality in the cell balancing circuit, and the additional two comparators for each diagnosis circuit, and thus, it has a disadvantage of increased manufacturing cost of the apparatus. Also, it is possible to determine whether there is an abnormality in a cell balancing circuit corresponding to each cell, but it is difficult to correctly recognize the cause of an abnormality when the abnormality occurs in the entire cell balancing circuit.